Recently, it has been proposed to form an active layer of a TFT (Thin Film Transistor) by use of an oxide semiconductor film of zinc oxide or the like instead of a silicon semiconductor film. A TFT including an oxide semiconductor film is referred to as an “oxide semiconductor TFT”. An oxide semiconductor such as zinc oxide or the like has a mobility higher than that of amorphous silicon. Therefore, an oxide semiconductor TFT can be operated faster than a TFT using amorphous silicon. In addition, an oxide semiconductor film may be formed by a sputtering method. Therefore, an oxide semiconductor TFT has an advantage of being applicable to a larger-screen display device than a TFT using polycrystalline silicon (see, for example, Patent Documents 1 and 2).
According to a sputtering method, a film is formed by use of a target. A magnetron sputtering method is especially widely used in mass production system. According to this method, a magnet is located on a rear surface of the target to generate a magnetic field on a surface of the target, so that plasma is converged to the vicinity of the surface of the target. A target is generally produced as follows. In (indium) or solder is used as a brazing material, and a target tile is bonded to a support (backing plate) formed of Cu (copper). As the production devices have been enlarged, the targets have also been enlarged. Patent Document 3 discloses a method for solving problems caused by such an enlarged target.